The referenced application Ser. No. 193,254, of Oct. 1, 1980, by the inventor hereof, discloses a circuit of this type having two similary poled switching transistors which are connected to bridge the terminals of a direct current supply source. If more than one lamp is operated in such a system, each one of the lamp operating circuits has its own series resonance circuit which is formed by respective ballast inductances and a capacitor. The respective lamp operating circuits and the associated series resonance circuits are connected in parallel.
The referenced application discloses a system which is suitable both for single-lamp as well as for multiple-lamp operation which includes a protective circuit so that, in case of malfunction for example upon non-firing or starting of a lamp, the transistor push-pull switch is disconnected, so that damage to the lamps or to the system is prevented. Preferably, the protective circuit includes a thyristor which is connected to the base of one of the transistors-for example one which is connected to the positive terminal of the direct supply, and then to the negative terminal of the direct voltage supply.
The protective circuit as described in the referenced application Ser. No. 193,254 provides operating voltages for the disconnect transistor solely from the filter capacitor of the d-c supply. This, for example, is an electrolytic capacitor. Such a filter capacitor is connected to the network filter circuit of the bridge rectifier. It has been found that the voltage on the filter capacitor changes with loading, and thus the switching function of the thyristor will depend on loading on the lamp. In case of malfunction, the direct current supply is loaded to a greater extent so that, if load interruption should cause such higher loading, which is frequently the case when the disconnect thyristor must become operative, the switching conditions under which the disconnect thyristor operates is impaired due to the lower holding current available therefor.
The drop in voltage on the filter capacitor of a direct supply network is due to the current limiting effect of the filter choke which is provided to remove ripple from the network supply. The filter capacitor is capable of supplying increased current into the circuit for a short period of time without substantial drop in voltage upon shedding of load during normal operating conditions, that is, still within the response period of the protective circuit. Consequently, the switching conditions are usually suitable for emergency switching. If, however, switching conditions occur, for example if one lamp is not connected or not properly connected during initial connection of the entire unit, no charge with increased voltage can build up on the filter capacitor so that, then, the protective circuit may not be able to respond due to insufficient energy supply thereto.